Solvents and other organic compounds can be removed from air by oxidation or combustion. The impurities are made to react with oxygen in the air and they are thereby converted to harmless water and carbon dioxide. A high temperature is normally required for the reaction to proceed. When the impurity concentration is higher than the so-called lower explosive limit, but still under the upper explosive limit, the heat developed during the reaction is sufficient to maintain a high enough temperature, once the reaction has been initiated by ignition.
For safety reasons, however, most normal industrial processes take care to remain comfortably below this explosive limit when air containing solvents or other combustible vapors are involved. This means that emissions of ventilating air containing impurities are virtually always below the explosive limit, usually far below.
Ignition is not sufficient to produce combustion of the impurities under these circumstances. The impurity content is too low for the reaction to proceed on its own. One way to produce combustion despite this is to heat the air to a temperature of 750-1000.degree. C. This can be accomplished with electricity or with the help of a gas or oil burner. However, this drives up energy costs, even if heat exchangers are often used to recover heat from the treated air and to use this heat for heating the incoming, not yet treated air.
One way to reduce the reaction temperature and thus the energy consumption is to let the reaction proceed with the help of catalysts. For example, contact with metals from the platinum group can provide a good reaction rate even at 200 or 300.degree. C. Catalysts consisting of various blends of metal oxides are also used. This field is quite large and new catalysts and ways of handling these catalysts to provide better catalytic activity are being developed constantly.
However, temperatures well above room temperature are still normally needed for the reaction rate to be satisfactory.